1. Field of the Invention
The present invention relates to a power supply system applied to a radio frequency circuit module, especially to a digitally controlled non-inverting buck-boost DC-DC converter system applied to a radio frequency circuit module. The pulse-skipping behavior is avoided by locking duty-cycle. Moreover, a reference voltage is modified through a reference voltage correction circuit. Thus a digital compensation circuit neglects errors of a DC output voltage and converges into a steady state. The output voltage oscillation behavior is further improved and a stable, wide range direct current is output to match requirements of adjustable voltage of a power amplifier in the RF circuit module.
2. Description of Related Art
Due to broader use of wireless electronics, more and more consumers concern that whether batteries inside the wireless electronic have a power saving setting. Generally, the power consumption of portable products including mobile phones, tablet computers, notebooks, etc. is related to the following three components: displays, antenna receivers and transmitters, and digital processors. In the antenna receivers and transmitters, the component that consumes most of the power is a power amplifier. The power amplifier available now can adjust the operating voltage. Thus a lot of energy is saved when a signal is not with the largest strength and the battery life is extended. For example, when the wireless electronic is closer to the base station, only a little bit increasing of the signal is required to solve the signal attenuation problem and transmit RF signals successfully. In power source selection, a buck-boost DC-DC converter that outputs higher/lower battery voltage is the best choice. In contrast, a conventional boost converter is unable to provide proper voltage when the power amplifier requires low power consumption and low voltage operation. This results in a great amount of power loss between the power converter and the power amplifier. Yet the buck-boost DC-DC converter can avoid such problem.
The buck-boost DC-DC converter includes four switches for the buck/boost operation by means of saving switching losses. Thus switching losses of the 4-switch converter may be double of those of a buck or boost converter and this is a significant shortcoming. The solution to overcome the shortcoming mentioned-above available now is buck/boost operation. The principle of the buck/boost operation is checking the values of output and input voltages so as to determine which mode the converter is operating in. Thus there are only two switches for control of the duty-cycle no matter which mode is used. The other two switches are kept on-state/off-state and there are no switching losses. Thus the number of switching times of the switches in each cycle is decreased, so as to decrease the switching losses. However, a problem of how to determine the timing of mode switching is derived from the buck/boost operation. When the mode switching is not working well, pulse-skipping occurs. Once the converter is operated within the range near the extreme values of the duty cycle, the duty cycle sent by the controller is quite unstable. A large jitter is generated randomly. Once the jitter occurs in the steady state, the voltage has changes and further large-scale pulse-skipping occurs. Unfortunately, the mode switching timing of the buck/boost operation may just fall on the position with the worst linearity of the duty cycle. When the pulse-skipping mentioned above occurs, unstable power output may lead to imprecise operating voltage of the power amplifier. The output voltage even falls to the voltage level being too low during the skipping process. This causes a partial distortion of the RF signals during the transmission process. This is also called duty-discontinuity.
When the reference voltage value the load end of the buck-boost converter falls within the mode switching range, the above duty-discontinuity occurs. That means the duty cycle is non-linear, discontinuous. The jitter duty cycle results in unstable output voltage so that the output voltage fails to converge to the reference value (voltage). The method for solving the unstable output voltage problem caused by duty-discontinuity during mode transition available now is duty-overlapping.
The method is to overlap the duty-cycle signals of the two modes (buck/boost) within a switching cycle to get an average result and create a voltage value that only a single duty cycle is unable to generate. However, the duty-overlapping method has following disadvantages: (a) In this method, buck/boost operation is run in turn within a switching cycle so that four switches of the converter all need to be switched. Thus the efficiency is reduced due to the switching losses. (b) The duty-overlapping must be run under a certain condition to ensure that the pulse skipping will not occur. In order to get an averaged and proper output voltage, there is a plurality of combinations. For example, when the condition of the output voltage required is M=0.98 (M=Vo/Vbat, wherein Vo is an output voltage while Vbat is an input voltage), the combination can be buck mode/92%+boost mode/4%, buck mode/90%+boost mode/6% or others so as to average out a precise output value. The same averaged output value is resulted from different duty cycle combinations of the two modes. Thus the duty cycle of the two modes may still have respective variations. The duty cycles must be locked in a fixed solution of a solution set so as to avoid the pulse slipping caused by different combinations of the duty cycles.
Thus there is room for improvement and a need to provide a novel buck-boost DC-DC converter system that improves the above shortcomings caused by duty-discontinuity.